The present invention relates to a method for the production of colorless polyester amides based on dicarboxylic acids and/or their esters, primary diamines, and polyhydric alcohols and/or aminocarboxylic acids or lactams, by polycondensing the mixture with heating in the presence of water while distilling off the volatile components, and recondensation under reduced pressure.
Polyester amides have been known for a long time. In principle, they are prepared by two different methods. The synthesis is accomplished either by the use of low molecular starting components, such as for example, aminoalcohols, lactones, diols, dicarboxylic acids, aminocarboxylic acids, lactams, and diamines, or by reactions between polyamides and polyesters, optionally in the presence of catalysts at elevated temperatures.
The state of the art of polyester amide production may be ascertained by reference to "Polyamides Other Than Nylons 6 and 66," Part II, Stanford Research Institute, Menlo Park, Calif., November 1974, particulary the List of Patents on Polyester Amides, pp, 417-422; U.S. Pat. Nos. 2,856,385 and 3,160,609; German Published Applications Nos. 1,745,448 and 2,236,041; and British Pat. No. 959,671, the disclosures of which are incorporated herein.
Polyester amides heretofore have hardly been used under practical conditions. This is essentially due to the fact that discolorations occur at temperatures of above 200.degree. C during longer heating of the starting materials and this discoloration is attributed to secondary reactions not examined in detail heretofore.
To avoid such discolorations, the following methods are known to the prior art:
1. The polycondensation temperature is lowered as disclosed in German Published Application No. 1,745,448. However, this procedure does not result in molecular weights of above 10,000 which are required for practical application.
2. The use of reactive catalysts. The critical temperature limit is exceeded, but the condensing periods are kept to a minimum. Examples of effective catalyst systems used are tetraphenyl tin as disclosed in U.S. Pat. No. 3,160,609, as well as alkoxy titanates as disclosed in British Pat. No. 959,671, and alkali hydrogen hexaalkoxy titanates as disclosed in U.S. Pat. No. 2,856,385. The brief reaction times, for example 30 - 90 minutes, cannot be maintained in the case of a discontinuous process in a stirred autoclave, since the heating times and the duration of the discharging of the polymer melt alone exceed the above time limits by far. Besides, residues of the product in the reactor and in the feed conduits, which are under multiple thermal load, strongly discolor the subsequent batches. This can only be prevented by thorough decocting after each batch and this represents a considerable loss in capacity and is connected with increased expenses.
3. The use of 2,2-dimethyl-1,3-propanediol (neopentyl glycol) as the diol component is disclosed in German Published Application No. 2,236,041. The production of the polyester amides is effected by melt condensation of diamines and dicarboxylic acids and/or .omega.-aminocarboxylic acids or lactams, preferably in the presence of catalysts and/or stabilizers and further additives, the diol employed being exclusively neopentyl glycol. The avoidance of discoloration is thought to be due to the structural peculiarity of neopentyl glycol (no H-atoms in the .beta.-position with respect to the OH-group), so that the thermal ester cleavage is impossible. When diols not endowed with this special structure are used, such as for example, ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,12-dodecanediol, 1,4-bis(hydroxymethyl) cyclohexane, unattractive, dirty brown products are obtained, the discoloration of which is intensified upon further thermal loading.